GRS 1758–258: RXTE Monitoring of a Rare Persistent Hard State Black Hole

M Obst,D M Smith,K Pottschmidt,I Kreykenbohm,J A Tomsick,J Wilms,M Böck,A Lohfink

doi:10.14311/1480

Abstract

GRS 1758–258 is the least studied of the three persistent black hole X-ray binaries in our Galaxy. It is also one of only two known black hole candidates, including all black hole transients, which shows a decrease of its 3-10 keV flux when entering the thermally dominated soft state, rather than an increase.We present the spectral evolution of GRS 1758–258 from RXTE-PCA observations spanning a time of about 11 years from 1996 to 2007. During this time, seven dim soft states are detected. We also consider INTEGRAL monitoring observations of the source and compare the long-term behavior to that of the bright persistent black hole X-ray binary Cygnus X-1. We discuss the observed state transitions in the light of physical scenarios for black hole transitions.

Highlights

GRS 1758−258 (Figure 1) is an intermediate mass Xray binary harboring a black hole and a companion consistent with an early A-type main sequence star, but with unusual colors [2]
), the source almost turned off completely. This strong decline in flux makes GRS 1758–258 especially interesting as it is typical for transient, not for persistent sources
As GRS 1758–258 is a rather faint source located close to the Galactic Center (Figure 1), the RXTEPCA spectra contain source counts and a strong background component caused by the Galactic ridge emission (Figure 3)

Summary

Introduction

GRS 1758−258 (Figure 1) is an intermediate mass Xray binary harboring a black hole and a companion consistent with an early A-type main sequence star, but with unusual colors [2]. Mass transfer is probably driven by Roche lobe overflow. Among such systems, usually transients, GRS 1758−258 is one of only few persistent sources. Usually transients, GRS 1758−258 is one of only few persistent sources It can be found in the hard state. In some respects it still displays a behavior typical for transient sources [3–6, hysteresis, rare decay-type soft states]

RXTE Monitoring

Background Modeling

Spectral parameters

Hardness intensity diagram

Summary and outlook